Heavy-duty rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, and the like. Such bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled.
While such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in such severe environments is on the order of 20 to 200 hours for bits in sizes of about 6 to 28 inch diameter at depths of about 5,000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the cutter cones are mounted. The journal bearings are subjected to very high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted over the years to produce improved bearing structures and bearing materials that minimize wear and failure of such bearings.
A variety of bearing compositions have been employed in the past. Bearing compositions which have been used include cast or wrought forms of copper-based spinodal composites such as disclosed in U.S. Pat. No. 4,641,976, the disclosure of which is expressly incorporated herein by reference. These bearing compositions comprise ternary alloys of copper with nickel and tin but may contain other metals to further improve the metallurgical properties.
In order to further enhance properties of bearing material various additions to alloys have been proposed. However, some elements otherwise desirable for addition to improve properties are insoluble or only slightly soluble in copper solid solution and will form compounds that segregate during melting and subsequent thermomechanical processing. Metalloids such as Sb, As, S, Sn, Se, Te, Be, P, etc., are examples of elements having, at best, limited solid solubility in copper but which form stable compounds that would usefully improve strength and toughness of a copper alloy matrix through a combination of several of the phase transformations such as solid solution strengthening, precipitation hardening and spinodal decomposition. However, these metalloidic elements are highly surface active in a copper matrix and tend to segregate at the high energy areas such as grain boundaries, dislocations and other crystal defects.
In view of the foregoing, it is evident that it is desirable to provide a bearing material for rock bits that is less susceptible to premature wear or failure during service at the high temperatures, bearing pressures and rotational speeds often found in modern rock bits that combines high strength and other desirable metallurgical properties through a combination of several phase transformations such as solid solution strengthening, precipitation hardening and spinodal composition without undesirable segregation of ingredients.